Walther Bothe, in full Walther Wilhelm Georg Bothe
(born Jan. 8, 1891, Oranienburg, Ger.—died Feb. 8, 1957, Heidelberg, W.Ger.), German physicist who shared the Nobel Prize for Physics in 1954 with Max Born for his invention of a new method of detecting subatomic particles and for other resulting discoveries.

Bothe taught at the universities of Berlin (1920–31), Giessen (1931–34), and Heidelberg (1934–57). In 1925 he and Hans Geiger used two Geiger counters to gather data on the Compton effect—the dependence of the increase in the wavelength of a beam of X rays upon the angle through which the beam is scattered as a result of collision with electrons. Their experiments, which simultaneously measured the energies and directions of single photons and electrons emerging from individual collisions, refuted a statistical interpretation of the Compton effect and definitely established the particle nature of electromagnetic radiation.

With the astronomer Werner Kolhörster, Bothe again applied this coincidence-counting method in 1929 and found that cosmic rays are not composed exclusively of gamma rays, as was previously believed. In 1930 Bothe discovered an unusual radiation emitted by beryllium when it is bombarded with alpha particles. This radiation was later identified by Sir James Chadwick as the neutron.

During World War II Bothe was one of the leaders of German research on nuclear energy. He was responsible for the planning and building of Germany’s first cyclotron, which was completed in 1943.

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